The present invention relates to a photoelectric cell, a coating liquid for forming a metal oxide semiconductor film for use in a photoelectric cell and a process for producing a metal oxide semiconductor film for use in a photoelectric cell. More particularly, the present invention is concerned with a photoelectric cell wherein the adsorption and carrying amounts of photosensitizer on a metal oxide semiconductor film are large and the strength of bonding of the photosensitizer to the metal oxide semiconductor film is large with the result that the photoelectric cell exhibits enhanced photoelectric transfer efficiency. Further, the present invention is concerned with a coating liquid for forming a metal oxide semiconductor film for use in such a photoelectric cell and a process for producing a metal oxide semiconductor film for use in such a photoelectric cell.
A photoelectric transfer material is a material from which light energy is continuously taken out as electric energy and a material which converts light energy to electric energy by the utilization of electrochemical reaction between electrodes. When the photoelectric transfer material is irradiated with light, electrons are generated from one electrode. The electrons move to a counter electrode, and the electrons having reached the counter electrode return by moving as ions through an electrolyte to the one electrode. This energy conversion is continuously carried out, so that it is utilized in, for example, a solar cell.
The common solar cell consists of an electrode formed by coating the surface of a support such as glass plate with transfer conductive film, wherein semiconductor for photoelectric transfer material is formed on the transfer conductive film, a counter electrode formed by coating the surface of a support such as a glass plate with another transparent conductive film, and an electrolyte sealed between these electrodes.
When this solar cell is irradiated with sunlight, the photosensitizer incorporated therein absorbs visible radiation region to thereby excite the electrons of photosensitizer dye. The excited electrons move to the semiconductor for photoelectric transfer material, and then the transparent conductive glass electrode, further move to the counter electrode. The electrons having reached the counter electrode reduce the oxidation-reduction system present in the electrolyte. On the other hand, the photosensitizer having caused electrons to move to the semiconductor becomes oxidized form. This oxidized form is reduced by the oxidation-reduction system (which becomes reduced form) of the electrolyte to thereby return to the original form. In this manner, electrons continuously flow. Therefore, functioning as the solar cell comprising the semiconductor for photoelectric transfer material can be realized by virtue of the continuous flow of electrons.
A semiconductor having a surface on which a photosensitizer exhibiting absorption in visible radiation region is adsorbed is used as such a photoelectric transfer material. For example, Japanese Patent Laid-open Publication No. 1(1989)-220380 describes a solar cell comprising a metal oxide semiconductor and, superimposed on a surface thereof, a layer of a photosensitizer such as a transition metal complex. Further, Published Japanese Translation of PCT Patent Applications from Other States, No. 5(1993)-504023 describes a solar cell comprising a titanium oxide semiconductor layer doped with metal ions and, superimposed on a surface thereof, a layer of a photosensitizer such as a transition metal complex.
In these solar cells, for increasing the photoelectric transfer efficiency, it is important that the move of electrons from the photosensitizer layer having absorbed light and having been excited to a titania film be performed rapidly. When the move of electrons is not performed rapidly, the electrons recombine with the transition metal complex such as a ruthenium complex to thereby invite the problem of causing a drop of photoelectric transfer efficiency. Therefore, studies, such as on the improvement of the condition of bonding of the photosensitizer to the titania film surface and the improvement of electron mobility within the titania film, are performed.
For improving the condition of bonding of the photosensitizer to the titania film surface, for example, it has been proposed to, at the time of forming a metal oxide semiconductor film, repeat the operations of application of titania sol to a base, drying and annealing to thereby form a porous thick film, thus rendering the semiconductor film porous, with the result that the amount of Ru complex carried on the surface is increased. Further, it has been proposed to carry out an annealing at 400° C. or higher to thereby sinter fine particles of titania with the result that a conductivity enhancement is realized. Still further, in Published Japanese Translation of PCT Patent Applications from Other States, No. 6(1994)-511113, it is carried out in order to increase the effective surface of the semiconductor to, after the formation of a semiconductor layer composed of a porous titanium oxide, immerse the semiconductor layer in an aqueous solution of titanium chloride, or to electrochemically deposit titanium oxide on the porous titanium oxide semiconductor layer with the use of a solution of titanium chloride hydrolyzate.
However, in the current state of the art, there are problems, for example, when annealing is effected for increasing the electron mobility, sintering occurs to thereby decrease the porosity (effective surface) with the result that the adsorption amount of photosensitizer is lowered. Moreover, because the photoelectric transfer efficiency is not satisfactory, the use thereof is limited. Therefore, there is a demand for further improvement of the solar cells.
An object of the present invention is to provide a photoelectric cell wherein the adsorption proportion of photosensitizer is high, the reactivity of photosensitizer is high, electron movement is smooth within the semiconductor and, hence, the photoelectric transfer efficiency is enhanced. Another object of the present invention is to provide a coating liquid for forming a metal oxide semiconductor film for use in such a photoelectric cell. A further object of the present invention is to provide a process for producing a metal oxide semiconductor film for use in such a photoelectric cell.